Reference is made commonly assigned copending patent application serial number, filed simultaneously herewith in the name of Amin et al. and entitled xe2x80x9cNarrow Band Wavelength Division Demultiplexer and Method of Demultiplexing Optical Signalsxe2x80x9d
1. Field of the Invention
The present invention generally relates to optical multiplexers and more specifically relates to wavelength division multiplexers.
2. Technical Background
Wavelength divisions multiplexers are used in optical communication networks to combine various optical signals (channels) carried by two or more optical wavelengths into a single, common carrier (for example, an optical waveguide such as a single fiber). Wavelength division demultiplexers are used in optical communication circuits to separate a plurality of signals transmitted on a common carrier based upon the wavelength of the light onto which the signal is modulated. Wavelength division multiplexers and demultiplexers typically include various combinations of optical elements for performing the combination and separation function respectively. The most common of such components are band edge dichroic filters, which reflect light having wavelengths above or below a certain characteristic wavelength into a first transmission path while allowing the remaining light (i.e., the light having wavelengths below or above the characteristic wavelength) to be transmitted through the band edge filter into a second transmission path. Such edge filters are not ideal in that they have a transition zone surrounding the characteristic wavelength. See, for example, FIG. 1, which shows the characteristics of a hypothetical ideal band edge filter F1 that does not exist or is otherwise extremely expensive to create, and FIG. 2, showing the characteristics of an actual band edge filter F2 as commonly used in these types of devices. Incident light having a wavelength in the transition zone (e.g., xcex4) is partially reflected and partially transmitted. When a band edge filter only partially transmits or reflects incident light that is supposed to be entirely transmitted or reflected, the band edge filter reduces the intensity of the light signal that is transmitted through the intended transmission path while introducing noise into the other path (i.e. transmission path). To avoid such signal loss and noise, either the separation between the channels must be large enough so that no channels fall within the transition zone of the filter, or the filter must be nearly ideal so as to have a transition zone smaller than the channel separation. To accommodate more signals on a single optical fiber trunk line, designers must decrease channel (i.e., wavelength) separation, which makes the non-ideal band edge filters less practical for use in a wavelength division multiplexers and demultiplexers.
The following description of prior art is directed to both multiplexers and demultiplexers, because these devices are similar to one another and generally a multiplexer will function as a demultiplexer when the input and output are reversed so as to separate (with a demultiplexer) instead of combining (with a multiplexer) different wavelength signals.
U.S. Pat. No. 5,652,814 issued to Pan et al. discloses a wavelength division demultiplexer made up entirely of such band edge filters. This demultiplexer is shown in FIG. 3. As illustrated, a first filter 271 reflects signals having wavelengths, xcex1-xcex4 while transmitting signals having wavelengths xcex5-xcex8. A second filter 272 receives signals having wavelengths xcex1-xcex4 and reflects signals having wavelengths xcex1 and xcex2 while transmitting signals having wavelengths xcex3 and xcex4. Similarly, a third filter 273 receives signals having wavelengths xcex5-xcex8 and reflects signals having wavelengths xcex5 and xcex6 while transmitting signals having wavelengths xcex7 and xcex8. Additional band edge filters 274-277 are provided as a final separation stage. Because the band edge filters are not ideal, the demultiplexer disclosed in Pan et al. would exhibit large levels of signal loss and crosstalk, particularly when the channel separation is small.
To overcome these difficulties, wavelength division demultiplexers have been constructed with wavelength channel dropping components that include a combination of an optical circulator and various fiber Bragg gratings (FBGs). An example of such a demultiplexer is disclosed in U.S. Pat. No. 5,754,718 issued to Duck et al. This demultiplexer is illustrated in FIG. 4. As shown at the left side of FIG. 4, eight channels having wavelengths xcex1-xcex8 are transmitted into port 1 of an optical circulator 610. All these signals are transmitted out of circulator 610 at port 2. These signals are then passed through the four FBGs that are configured to reflect the signals of non-adjacent wavelengths xcex2, xcex4, xcex6, and xcex8 back into port 2 of optical circulator 610. The remaining non-adjacent wavelengths are transmitted into port 1 of a second optical circulator 612. First circulator 610 transmits the signals of wavelengths xcex2, xcex4, xcex6, and xcex8, which are reflected into port 2, out of port 3. Two FBGs reflect wavelengths xcex2 and xcex6 and provided at port 3 of optical circulator 610, reflect signals having wavelengths xcex2 and xcex6 back into port 3 of optical circulator 610. Optical circulator 610 transmits these signals from port 4. Signals of wavelengths xcex4 and xcex8, however, which exit port 3 of circulator 610, are transmitted through the FBGs to a band edge filter 626. Band edge filter 626 transmits signals of wavelength xcex4 and reflects signals of wavelength xcex8 (signals xcex5, xcex6, and xcex7 are already removed from the optical path). Similarly, a band edge filter 628 separates signals of wavelengths xcex2 and xcex6 which exit port 4 of circulator 610. Optical circulator 612 and band edge filters 622 and 624 similarly separate the signals of wavelengths xcex1, xcex3, xcex5, and xcex7. As will be apparent, the wavelengths of the signals transmitted to each of band edge filters 622-628 are not in adjacent channels. Therefore, any transition zone present in band edge filters 622-628 does not necessarily degrade the strength of the signals or the ability to separate the signals based on their wavelengths.
While the wavelength division demultiplexer shown in FIG. 4 overcomes the above noted problems relating to channel separation using band edge filters, the construction of such a demultiplexer is quite expensive due to the very large number of FBGs and other necessary optical components. In addition, the demultiplexer shown in FIG. 4 is designed to separate eight optical channels. If the number of channels to be separated were increased, the demultiplexer would need to be substantially redesigned.
U.S. Pat. No. 5,748,350 is directed to both wavelength division multiplexers and demultiplexers. The multiplexers are illustrated, for example, in FIGS. 1A, and 6A of this patent. FIGS. 7A and 7B illustrate 4nX1 wavelength division multiplexers. FIGS. 8A and 8B of this patent illustrate 4nX1 wavelength division demultiplexers, while FIGS. 9, 10A and 10B show devices that function as wavelength division multiplexers and demultiplexers. These multiplexers and demultiplexers utilize multiport optical circulators, fiber Bragg gratings and band pass wavelength division couplers. The multiplexers disclosed in this reference require fiber Bragg gratings for more than 50% of the optical channels, and, typically, optical bandpass filters for each of the optical channels to be multiplexed. That is, a separated bandpass coupler is required for each channel.
As stated above, an exemplary multiplexer is shown schematically in FIG. 6A of that reference. In this multiplexer the multiple fiber Bragg gratings are inserted directly after the optical circulator and are used to direct particular wavelengths of light to specific ports of the optical circulator. Different wavelength channels transmitted by the specific circulator ports interleaved, as shown in FIGS. 2F and 2G, so that the spectral edges of the bandpass devices will not coincide with any optical channel to be multiplexed. The twelve channel multiplexer described in FIG. 6A utilizes 21 optical components. These components are: one optical circulator, twelve filtering elements, and eight fiber Brag gratings. Because every optical component introduces impairments (including optical and polarization dependent losses) into the optical system, it would be desirable to minimize the number of optical components and, therefore to increase the efficiency of multiplexers and demultiplexers.
It is an object of the present invention to provide a wavelength division multiplexer that is less expensive to manufacture than most prior systems and that has little, if any, signal loss. To achieve these and other objects and advantages, the wavelength division multiplexer of the present invention comprises a channel combining component receiving optical signals transmitted through a plurality of optical channels. The channels are defined by successively different light wavelength bands at intervals ranging between a first channel having the lowest wavelength band to a last channel having the highest wavelength band. The channel combining component combines at least one channel having a wavelength band intermediate the lowest wavelength band and the highest wavelength band with channels having the lowest and the highest wavelength bands. The channel combining component includes an edge filter. This edge filter combines optical signals in the highest and lowest wavelength bands, but not in the intermediate wavelength band. (The highest and said lowest wavelength bands are situated respectively above and below the intermediate wavelength band.) The edge filter provides the combined optical signals to the channel combining component.
Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description, or recognized by practicing the invention as described in the detailed description which follows the claims, as well as the appended drawings.
It is to be understood that both the foregoing general description and the following detailed description are merely exemplary of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate various features and embodiments of the invention, and together with the description serve to explain the principles and operation of the invention.